It is well known that in order to maintain reliable communications between an orbiting satellite and ground stations, the antenna of the satellite system must be pointed accurately toward the ground station antenna with which the satellite is in communication using a high-gain reflector antenna system. In order to achieve this accurate pointing, satellites commonly employ tracking systems to provide signals indicative of the pointing errors in elevation and azimuth relative to the antenna beam of the ground station antenna. These tracking signals control the satellite's reaction control system to orient the satellite as required to position the antenna accurately towards the ground station antenna despite changes in the relative locations thereof. Typically, there is a corresponding tracking system at the ground station that permits accurate pointing of the ground station antenna as well.
Typically, the tracking system on the satellite utilizes a monopulse-tracking configuration in which a plurality of antennas, feeding a reflector-system, are employed to develop three tracking signals indicative of the pointing accuracy of the satellite antenna. These three tracking signals are the azimuth difference signal, elevation difference signal, and the sum signal. The phase and amplitude characteristics of these three signals are utilized in a conventional manner to generate elevation angle error and azimuth angle error signals to control the pointing direction of the satellite antenna. The specific manner in which the monopulse tracking receiver operates is well-known in the art and need not be described in detail herein. By way of example, the use of monopulse tracking systems for radar applications is treated extensively in the text entitled Radar Handbook by M. I. Skolnik, published by the McGraw Hill Book Company in 1970.
One disadvantage of conventional monopulse tracking systems is that such systems are designed to operate with cumbersome antenna arrays. In such arrays, a plurality of antennas are used to develop the sum and difference signals needed to provide the receiver with the means for developing the elevation and azimuth angle error signals for controlling the tracking system. Such cumbersome plural antenna arrays tend to be larger and heavier than desirable for satellite applications. In addition, because the beam of each such antenna is located at discrete point separated from the beam of each of the other antennas in the array, monopulse tracking with such a system tends to introduce inherent tracking errors that reduce the accuracy of the tracking system. Too small a separation distance between feed antennas reduces the antenna system efficiency. Too large a separation distance between feed antennas places the beam cross-over points in the respective sidelobes of the beams rendering the antenna system highly susceptible to instability errors. These problems are exacerbated further in those satellite tracking systems that employ different uplink and downlink frequencies for dual mode tracking and communication.
The present invention comprises a feed system that overcomes the disadvantages of the prior art mentioned above by providing the monopulse sum and difference signals for a monopulse tracking receiver while operating with surprisingly efficient mode coupling in conjunction with only a single antenna. In addition, the present invention makes it possible to efficiently utilize that single antenna for downlink transmission as well.
An additional advantage of the present invention relates to the polarization of the electromagnetic energy transmitted between ground station and satellite. More specifically, in conventional monopulse tracking systems for satellite applications, circular polarization is used for the tracking signal to minimize inadvertent tracking errors that might otherwise occur when such monopulse systems are implemented with multiple antenna arrays. However, at the very high frequencies of of transmission of modern satellite communication tracking systems, such as at frequencies above 15 GHz, studies have shown severe degradation of the propagation of such circularly polarized high frequency signals as a result of heavy rain. Consequently, for certain applications such as highly accurate tracking, the use of circularly polarized signals may not be feasible with consistent reliability. The present invention also circumvents this rain-induced signal degradation problem by using linear polarization to derive the tracking error signals as well as the uplink and downlink sum signals as will be more fully understood hereinafter. The highly efficient use of a single antenna feed system, made possible by the present invention, results in a more efficient transmission link which overcomes the reduction in transmission efficiency that arises in use of linear polarization. STATE OF THE PRIOR ART
There are numerous patents which disclose coupling concepts that are relevant to the present invention. By way of example, U.S. Pat. No. 3,731,236 to DiTullio discloses a system coupled to a single antenna horn which includes means for handling two independently polarized signals at one frequency in combination with a second means isolated from the first means by a cut-off which is capable of processing two independent polarized signals at a second frequency.
U.S. Pat. No. 3,369,197 to Giger et al discloses a satellite tracking system incorporating a single antenna feed horn in combination with coupling means capable of isolating several modes of propagation of circular polarization.
U.S. Pat. No. 3,566,309 to Ajioka discloses means for coupling four waveguide modes representing two different frequencies from a horn antenna and a tracking system.
U.S. Pat. No. 3,715,688 to Woodward discloses the concept of utilizing slots which function as grids which assist in creating a TM.sub.01 mode and linearly polarized TE.sub.11 mode.
U.S. Pat. No. 2,730,677 to Boissinot et al discloses a concept of extracting energy from a circular waveguide segment by means of two rectangular waveguide segments.
Other multi-mode, single antenna feed systems using relatively inefficient coupling schemes are disclosed in articles appearing at pages 62 et seq of the 1962 NEREM Record and at pages 94 et seq of the 1963 NEREM Record, respectively. These two articles are respectively entitled: Feed Design For Large Antennas by Jensen et al, and A Low-Noise Multimode Cassegrain Monopulse Feed With Polarization Diversity by Jensen.
However, none of the known prior art discloses a device utilizing the high efficiency coupling scheme of the present invention for using linear polarization to derive the tracking error signals and sum pattern for a monopulse tracking system from one antenna at a single receiving frequency. Furthermore, applicants know of no prior art which, in addition to the above, also provides means for transmitting at a different frequency utilizing still an additional mode of waveguide operation and linear polarization.